I found this interesting, so I went to the mfg website found the following
the accuracy of +/- 5% is a worry, but not sure what exactly they mean by the second statement about being calibrated to a known source?
The WS-05 includes adjustable temperature and humidity for the upmost accuracy. Calibration is not required and the accuracy specification of +/- 1 degF for temperature and +/- 5% for humidity may be perfectly acceptable for most users, but for critical applications, the temperature and humidity can be adjusted to match a known, calibrated source.